1. Field of the Invention:
The invention relates to a blow out preventer which may be utilized within a hydraulic circuitry extending from a surface control panel to a safety valve located within a subterranean well.
2. Description of the Prior Art:
In order to isolate a subterranean oil or gas well during a blow out, fire or other catastrophy, safety valves are installed in the well, which may be manipulated between open and closed positions by variation of hydraulic pressure contained within a line extending from the safety valve to a control panel at the surface of the well. Typically, such control systems have utilized an hydraulic-pneumatic interface valve which is sensitive to variance within a pneumatic signal source which depicts abnormal well conditions, such that the pneumatic signal will activate the interface valve to send hydraulic pressure to the downhole safety valve and maintain the safety valve in an open position until an adverse well condition causes sufficient variation within the pneumatic signal source to, in turn, cause the interface valve to shift and vary the hydraulic pressure signal to the safety valve to manipulate it to a closed position. As the hydraulic-pneumatic interface valve is activated, the interface assembly shifts to block hydraulic supply and concurrently bleeds pressure out of the control line extending to the downhole safety valve. However, such assemblies having a hydraulic-pneumatic valve within the control panel permit a continuous communication of fluid within the control line to the reservoir after the shut-in signal has been activated, because the dump port within the hydraulic-pneumatic interface valve is opened and remains opened, even though the downhole safety valve has been manipulated to the closed position. In the event of a packing or conduit failure immediate the safety valve assembly, the open port within the hydraulic-pneumatic interface valve assembly will permit well and control fluids to flow up through the control line and into the control unit. There is often a potential fire hazard and the environmental pollution possibility is readily apparent.
It has been previously proposed, for example in U.S. Pat. No. 4,202,368 to Akkerman et al., to provide a blow out preventer wherein the closing manipulation of the safety valve does not interfere with utilization of the control line blow out preventer disclosed in such patent. Such patent provides a blow out preventer that permits repeated operation of the safety valve between open and closed positions. Only when flow returns at excessive flow rates are encountered within the blow out preventer described in the aforementioned patent during an abnormal closing of the safety valve, or during emergency shut down of the well, does the previously patented blow out preventer effect the isolation of the control line from the blow out preventer to the safety valve from the hydraulic circuitry extending to the control panel.
Utilization of the blow out preventer device disclosed in the aforementioned Akkerman et al. patent has revealed that under cetain unique conditions, the blow out preventer may not reliably effect the closing of the ball valve which shuts off communication between the downhole safety valve and the blow out preventer. The unique conditions that would cause this undesired operation is the presence of gas in the control line extending from the blow out preventer to the downhole safety valve. When the control panel is operated to remove fluid pressure from the control line extending to the safety valve through the blow out preventer mechanism, any trapped gas contained in the line intermediate the blow out preventer and the downhole safety valve will effect a very rapid shifting of the piston contained in the blow out preventer to its upper position due to the fact that there is little, if any, pressure opposing the rise of the piston, and the trapped gas is applied to the underside of the piston and effects its rapid movement to the uppermost position. In such uppermost position, the actuating stem for the ball valve, which permits the closing of such valve when the piston reaches its upper position, is free to retract, but, because of the trapping of gas pressure below the piston, such pressure opposes the closing of the ball valve and hence it may remain open for a significant period, thus, defeating the purpose of the blow out preventer.
Additionally, there is need for a temperature sensitive actuation of the ball valve that is normally positioned between the blow out preventer and the conduit extending to the downhole safety valve. A blow out or fire may occur near the well head and its presence may not be reported by other instrumentation to the control panel to trigger the operation of the control panel to close the downhole safety valve. There is a need for a mechanism for effecting the closing of such ball valve in response to abnormal temperature conditions, even prior to the shifting of the main piston of the blow out preventer to its upper or flow disconnecting position.